What is the plastic piece under the front of my car for?

I assume you mean the large flat plastic shroud under the front of the car. Basically it is a water and debris deflector to protect the components from excessive water, branches, twigs, garbage and other debris from hopping up into the engine compartment.

OR ...do you mean the thing that looks like a scraper blade that sticks down below and back a little ways from the front of the bumper?... Any problems with the engine temperature if it's not on there??